This information tells you how much did the light from your flash unit contribute to the total exposure. This info is helpful if you use external flash to exposure the subject in your photograph. Example of the same scene with different Flash vs. Ambi ratio:
Lumu Power is an incident light meter, so when measuring Ambi or Flash Exposure you have to have it placed close to your subject end keep the Dome faced directly towards the camera lens.
Of course, if you are measuring a person you can have Lumu Power placed
- directly in front of the person or
- on the left or right side of his/her face.
It depends on which part you want to have properly exposed and what your desired effect is.
When shooting Flash, you have 4 variables:
- Exposure Time
- Flash power output.
All 4 variables contribute to the final exposure. But be aware that only two things impact the Flash vs. Ambient ratio: changing the Flash power output or changing the Exposure time.
Changing ISO and Aperture will only make sure your photo isn’t in generally over- or underexposed.
See this picture with three different scenarios:
For further reading we suggest you take a look at this series of articles by David Hobby, who is master in flash photography http://strobist.blogspot.si/2006/03/lighting-101-balancing-flash-and.html.
His general advice is:
- Read ambient exposure first (without flash). This will give you a good impression about the available light and where you stand with Exposure time/ISO/Aperture.
- Drop the exposure down to create a little "drama". How much, is up to you.
- Bring your subject back up to full exposure by adding flash.
- Then play and experiment.